1. Field of the Invention
The present invention relates to a musical tone synthesizing apparatus which is suitable for simulating sounds of plucked string instruments, struck string instruments and the like.
2. Prior Art
Conventionally, apparatuses are known which synthesize musical tones of natural musical instruments by simulating their sound generation mechanisms. As for musical tone synthesizing apparatuses which synthesize the sounds of stringed instruments, there is the well-known apparatus which contains a closed-loop, including a low-pass filter for simulating reverberation-loss of strings and a delay circuit for simulating transmission delay of vibration of the strings. Alternatively, the apparatus may contain a multi-stage FIR filter (i.e., Finite Impulse Response digital filter) for simulating the string vibration. In the above-mentioned apparatus, when an excitation signal such as an impulse signal is supplied to and circulates in the closed-loop, the period in which the excitation signal circulates through the closed-loop once is set equal to the vibration period of the string. Furthermore, the frequency band-width of the excitation signal is limited when it is transmitted through the low-pass filter. Then, the signal circulating in the closed-loop is picked up as a musical tone signal.
Hence, in the above-mentioned musical tone synthesizing apparatus, by adjusting the delay time of the delay circuit and the characteristic of the low-pass filter, it is possible to generate musical tones which are approximately similar to the sounds of plucked string instruments such as the guitar or struck string instruments such as the piano. For example, the above-mentioned apparatus is disclosed in Japanese Patent Laid-Open Publication No. 63-40199 and Japanese Patent Publication No. 58-58679.
In acoustic instruments such as the piano, a hammer strikes a string so that the string vibrates at a pre-specified frequency. In this case, an acoustic instrument contains a predetermined number of keys, hammers and strings, wherein each pair consisting of a hammer and a string corresponds to each key. With respect to each key, the length and tension of the corresponding string and the inertia mass, the shape and solidity of the corresponding hammer are different from each other. For this reason, with respect to each key, there is a slightly different repulsion force which is applied to the hammer from the string when the hammer strikes the string. For example, a hammer corresponding to the key of relatively lower pitch has relatively rounder shape and larger mass, and the felt thereof at its striking point is formed relatively thicker and softer as shown in FIG. 12 (a). In contrast, a hammer corresponding to the key of relatively higher pitch has relatively sharper shape and smaller mass, and the felt thereof at its striking point is formed relatively thinner and more solid as shown in FIG. 12 (b). That is to say, in an acoustic instrument, each key has a different tone color depending on the repulsion force and the like applied to each hammer. For example, the sound of lower tone pitch has relatively softer tone color, while the sound of higher tone pitch has relatively harder tone color. However, according to the above-mentioned musical tone synthesizing apparatus, once data representative of the inertia mass and initial velocity of the hammer are inputted to the closedloop, such data settle the variation of the transition speed of the signal circulating in the closed-loop on a time axis. Hence, this apparatus cannot synthesize with high-fidelity acoustic musical tones the tone color of which is slightly different with respect to each key (or each tone pitch).